An avian RNA tumor virus which causes a high incidence of osteopetrosis will be characterized. Oligonucleotide mapping of the genome RNA will be used to establish the relatedness of this virus to others within the avian leukosis complex. Lymphocyte transfer experiments will be continued, especially those dealing with the effect of nonimmune lymphocytes on virus-induced bone growth. We will also examine whether lymphocytes from agammaglobulinemic donors are capable of protecting reconstituted infected chicks.